export> stl> EXPORT STL, blender keeps saving the wrong file
STL format only supports triangles.


  STL file format wiki
  
  STL (an abbreviation of "stereolithography") is a file format native to the stereolithography
  CAD software created by 3D Systems. STL has several
  backronyms such as "Standard Triangle Language" and "Standard
  Tessellation Language".
  
  An STL file describes a raw, unstructured triangulated surface by the
  unit normal and vertices (ordered by the right-hand rule) of the
  triangles using a three-dimensional Cartesian coordinate system. 


A simple test would be to export and re-import cube or plane, and note the triagulation.

Related Triangles to quad for 3D printing